This invention relates in general to a method of controlling engine output torque delivered to an automatic transmission for an automotive vehicle. More particularly, this invention relates to a method of controlling engine torque to minimize inertia torque disturbances when the transmission""s forward clutch is engaged upon the initiation of vehicle acceleration during launch from neutral idle operation.
Neutral idle operation of a vehicle is generally characterized by (i) the vehicle at rest, (ii) service brakes applied, (iii) gear select lever in a forward range, and (iv) all combinations of torque transmitting clutches that establish a speed ratio from the input to the output member of the transmission being disengaged. Therefore, neutral idle operation is further characterized by the freely rotating transmission input shaft rotating at substantially synchronous speed with the engine output shaft. Such neutral idle operation is of course beneficial, for example, to reduce engine fueling requirements by unloading the engine at zero vehicle speed idle conditions.
When a vehicle begins moving after being held stationary, it is said that the vehicle is launching from neutral idle operation. One method of launching from neutral idle operation is for the vehicle operator to merely cease applying the service brakes, at which point the vehicle begins to slowly creep ahead. Commonly, however, a vehicle operator will also apply the accelerator pedal soon after releasing the service brakes. Aggressive accelerator pedal applicationxe2x80x94that is various combinations of (a) the time between release of the service brakes and the accelerator pedal application, (b) the magnitude of the accelerator pedal application, and (c) the rate of the accelerator pedal applicationxe2x80x94may result in poor shift quality. Poor shift quality may include such effects as large output torque disturbances, undesirably short and harsh shifts, and undesirably long shifts with associated high on-coming clutch heating and premature wear. The poor shift quality is a result of the engine torque demanded by the vehicle operator exceeding the torque capacity of the transmission""s forward clutch while it is being engaged.
When a vehicle operator depresses the accelerator pedal from a neutral idle operation, engine torque output is demanded. As a result, the transmission""s forward clutch begins to engage in order to transfer the demanded engine torque through the transmission. However, a certain amount of time is required to pressurize and fully engage the forward clutch to enable its full torque transfer capabilities. During this time period (the xe2x80x9claunch periodxe2x80x9d), if the engine supplies torque in excess of the forward clutch""s torque capacity, the forward clutch may slip, and the driver may experience uneven vehicle response.
One method that has been developed to address the problems associated with undesirable slippage of the forward clutch is described in U.S. Pat. No. 5,795,262, herein incorporated by reference. According to the method described in the ""262 patent, engine torque output is controlled by limiting the actual amount of torque supplied by the engine during the launch period to a constant upper limit. Specifically, the engine torque output during the launch period is limited to the lesser of the torque requested by the vehicle operator and the constant upper engine brake torque limit. To ensure that the actual amount of engine torque output never exceeds the capacity of the forward clutch, the constant upper engine brake torque limit must be set based on the minimum clutch capacity. The inventors herein have recognized a disadvantage with this approach, namely the torque capacity of the transmission""s forward clutch increases throughout the launch period. Therefore, while the conventional method of imposing a relatively low constant limit on the engine torque output prevents engine flare, it fails to maximize the available torque capacity of the forward clutch as it increases throughout the launch period. As a result, the conventional method of minimizing engine flare causes relatively sluggish performance of the vehicle""s powertrain until the forward clutch is completely engaged.
The object of the present invention is to improve engine torque control during vehicle launch from neutral idle operation. The above object is achieved and disadvantages of prior approaches are overcome by a method for controlling a vehicle having an engine coupled to a transmission through a clutch, comprising determining an engine torque limit based on clutch torque capacity during launch from neutral idle operation, and adjusting an engine operating parameter to limit output engine torque to said engine torque limit.
According to the present invention, an engine brake torque limit is determined at various times throughout the launch period and until the forward clutch is fully engaged. Unlike the prior art, the engine brake torque limit varies throughout the launch period. Each time the engine brake torque limit is determined, it is compared to the engine torque requested by the vehicle operator at the corresponding time. Then, a microprocessor limits the actual engine torque output supplied at that particular time according to the lesser of the engine brake torque limit and the operator-requested engine torque.
The engine brake torque limit can be determined according to a variety of methods. One preferred method is to determine the engine brake torque limit at any given time based upon a pre-determined function. Specifically, because the torque capacity of the transmission""s forward clutch increases with time, the engine brake torque limit can be determined purely as a function of time. Therefore, as the elapsed time since the end of the neutral idle operation increases, so does the engine brake torque limit. Another preferred method to determine the engine brake torque limit is to estimate the torque capacity of the forward clutch at any given time using a mathematical model. Using this second preferred method, the torque capacity of the forward clutch is estimated at various times based upon certain operating parameters, such as clutch pressure. Then, an engine brake torque limit is determined based on the estimated torque capacity of the forward clutch and a calibrated positive or negative offset of turbine torque.
Irrespective of the particular method used, the determined engine brake torque limit is compared to the corresponding engine torque amount that is requested by the vehicle operator. Throughout the launch period, a microprocessor controls engine operating conditions, such as engine air/fuel ratio and engine spark, to limit the actual engine torque output according to the lesser of the determined engine brake torque limits and the corresponding operator-requested engine torque. As a result, the actual engine torque output is optimized at all times to provide approximately the maximum amount of requested torque without exceeding the capacity of the transmission""s forward clutch.